A 58-year-old man from rural India presents with progressive dysphagia and weight loss over 6 months. Upper endoscopy reveals a poorly differentiated adenocarcinoma of the gastric antrum with invasion into the muscularis propria. CT staging shows no distant metastases. He undergoes subtotal gastrectomy with D2 lymphadenectomy. What is the drug of choice for adjuvant chemotherapy in this patient?